Various types of personal computers (“PCs”), such as desktop and notebook (“laptop”) computers, are widely manufactured and sold today. The notebook PCs are compact and light to permit them to be portable. One of the features of notebook PCs is that they are “battery operable”, i.e., they can be driven by an incorporated battery. Such a system can be used at sites where there are no commercially available power sources. A battery that is incorporated in a notebook PC is commonly formed as a “battery pack”, which is a package that comprises a plurality of rechargeable battery cells (also called a “secondary cell”), such as Ni—Cd, NiMH, or Li-Ion. Although such a battery pack is reusable by being recharged, the battery duration is sufficient to supply power for only about two to three hours of system operation time. Therefore, various ideas for power saving have been implemented to extend the time between charge periods for a battery. The introduction of a power saving function can constitute another feature for the notebook Pcs.
At present, from an ecological point of view, the demand for power saving has increased even for desktop PCs that can be powered almost endlessly by commercially available power sources. And in June 1993, the U.S. Environmental Protection Agency (EPA) advocated the self-imposed regulations called the “Energy Star Computer program”, and required that power consumed in the standby state be lower than a predetermined value (driving power is to be 30 W or less, or 30% or less than it is when the CPU is active). Computer makers have developed and manufactured products that conform to the suggested regulation. For example, desktop PCs that have a power saving function are already sold by IBM Japan, Ltd. (e.g., the PS/55E (for which “Green PC” is a common name), PC 750, and the Aptiva series (“Aptiva” is a trademark of IBM Corp.)).
Power saving with a PC can be accomplished by, for example, reducing operational power consumption by the individual electric circuits in a system. Power savings can also be provided by reducing or halting, as needed, the power supply to the individual electric circuits (or peripheral devices) in the system in accordance with the reduction of their operational state (activity). The latter power saving function may especially be called a “power management” function.
As is well known, CPU chips are the units that constitute the nuclei for the computations that are performed by computer systems. Recently, as production techniques for manufacturing semiconductor devices have improved, as is demonstrated by the reduction in the wiring width, the operating frequencies of CPUs have increased even more.
For example, there have appeared CPU chips, such as the “Pentium” (a trademark of Intel Corp.) processors and the “PowerPC” (a trademark of IBM Corp.) processors, that can be driven at operating frequencies that exceed 1 GHz. The performance of a CPU and its operating frequency are very closely related. And as the operational speed of a CPU rises, the speed at which it performs calculations increases accordingly. A fast CPU demonstrates its excellent capabilities especially when running large application programs or when performing graphics procedures.
But, the high processing speed of the CPU brings with it several problems. One of the problems concerns the increased power consumption by the CPUs and the consequent heat generation. As the magnitude of a current that flows across a transistor gate (which is resistive) per unit time increases, the power consumption and the heat generation also increase. Theoretically, the power consumption by a CPU is proportional to the operating frequency. Currently, the ratio of the power consumption by a CPU to the total power consumption by the system cannot be ignored.
The appearance of electronically controlled vehicles controlled by so-called “electronic control units” (ECUs) comprising a microcomputer has increased drastically in recent years. In addition to control of the rotational speed of the internal combustion engine, control of gear changeover in a transmission and control of a clutch, these vehicles also have various accessories controlled by the ECU. Based on signals from various sensors provided on a variety of actuators, which drive devices to be controlled, the ECU calculates control variables for the various actuators that are controlled and then outputs the corresponding signals to these actuators to control the operation of each device.
Control systems of this type are used, for example, in motor vehicles for performing control functions which are typically found in vehicles.
Together with the trend toward increasingly electronically implemented functions in motor vehicles and their increasing mutual coupling, a significant rise in complexity occurs, along with a corresponding difficulty in the development and mastery of the entire electronic system of the vehicle. Additionally, this leads to a rising demand for computing power and memory capacity, as well as for an improved power management. Especially important is the need for a System-On-A-Chip (SOC) solution using on chip multi-control-element structure.
It is therefore an object of the present invention to control the performance and power requirements of Electronic Control Units.
It is another object of the present invention to provide such control in System-On-A-Chip (SOC) solutions.
It is still a further object of the present invention to save energy while still providing the optimal system performance as required by the application(s) being executed.